There are uses in many fields for sheet material with particular properties which result from the presence of particulate matter in and, specifically, at the surface of the sheet material. In particular, sheet materials with abrasive particles at the surface are available in the form of rolls or pre-cut sheets of sandpaper or in the form of abrasive discs and belts, and are used to provide anti-slip surfaces for walkways and similar locations. Also, for example, sheet materials with magnetizable particles at the surface are used as magnetic recording media; and sheet materials with reflective particles at the surface are used for pavement marking purposes and for signs. All of those sheet materials are typically produced by comparatively complex coating processes.
Abrasive sheet materials, for example, are conventionally produced by a coating process which typically involves the steps of applying an adhesive to a substrate; depositing abrasive grit particles on the adhesive (optionally using an electrostatic coater to orient the particles); drying and/or curing the adhesive; and, optionally, applying a size coating to anchor the grit particles more firmly in place. The coating process is comparatively slow because it involves multiple coating steps together with long drying and/or curing times, and careful control of the coating conditions is required to ensure the continuous output of an acceptable product. The process may involve the use of organic solvents, the disposal of which should be carefully controlled to reduce the risk of damage to the environment. Moreover, the process does not readily accommodate changes in the various coating materials when it is required to produce a different grade of product.
As an alternative to that process, it has been proposed to produce abrasive products by embedding abrasive grit in the softened surface of a substrate. U.S. Pat. No. 2,712,987 describes a process for making an abrasive belt, in which abrasive particles are distributed over the surface of a nylon substrate softened with a suitable solvent so that the particles become embedded, following which the solvent is dried. U.S. Pat. No. 2,899,288 describes a process for making an abrasive product, in which the upper surface of a thermoplastic backing sheet is softened by heating before abrasive material is spread over the surface and pressed into the sheet between rollers. U.S. Pat. No. 3,813,231 describes a process in which abrasive grit is distributed over the surface of a copolymer film which is then heated in a platen press to bond the grit to the film. U.S. Pat. No. 4,240,807 describes a process in which the substrate is paper coated with a heat-activatable binder which is softened by heating; abrasive grit is then distributed over, and allowed to sink into, the softened binder.
Polymer-based materials in the form of sheets are also known and can be produced simply and economically by extrusion, either through a blown film die or through a slot die. Slot dies can be used to produce comparatively thick cast films and also to coat a film onto a substrate. The extrudate from a slot die may be stretched in the longitudinal and transverse directions (tentered) to produce a film which is biaxially oriented. A blown film die is a ring die with a central passageway through which a required volume of air is admitted into the centre of the extruded polymer tube, causing the extruded tube to expand substantially to form a so-called "bubble". When the polymer has cooled, the expanded tube is collapsed and can then be cut to provide sheeting material of the required shape and size. A method and apparatus for producing blown film are described in, for example, U.S. Pat. No. 5,104,593.
It is known, when using both blown film dies and slot dies, to coextrude two or more polymeric materials simultaneously in a single die to form a multilayer sheet (see the "Encyclopaedia of Polymer Science and Technology", Volume 7, page 106 et seq., published in 1987 by John Wiley and Sons). Sheets produced in that way are typically used as packaging materials. In addition, it is known to use a blown film die to coextrude a polymeric backing layer and an adhesive coating to produce adhesive strips, as described for example in GB-A-1 553 881; U.S. Pat. Nos. 5,124,094; and 5,128,076. The use of a coextrusion process to produce a film base which is then coated with magnetizable particles to form a magnetic recording medium is described in U.S. Pat. No. 4,603,073. In that process, one of the extruded layers can include carbon black beads so that the film base will be opaque.
A process which involves the extrusion of a polyurethane material, an epoxy resin and an aggregate, to produce an anti-slip tread, is described in GB-A-2 268 748. It is also known to add a mineral, for example, calcium carbonate, as a reinforcement to a polymer which is to be extruded through a blown film die (see Ruiz and Bankole "Mineral Reinforcement of Linear-Low Density Polyethylene Film, Bags and Liners", conference proceedings of the Society of Plastics Engineers, 1992) or to add a pigment such as titanium dioxide (see Duchesne, Schreiber, Johnson and Blong "New Approaches to the Processing of Rutile-Filled Polyolefins", Polymer Engineering and Science, August 1990). JP-A-61 112633 refers to a process in which glass beads are injected into the interior of the expanded tube and the tube, when subsequently collapsed, is pressed between a pair of nip rollers to form a composite film.